Henry Fonda as Tom Joad in John Ford's The Grapes of Wrath (top); Fonda and Katharine Hepburn in Mark Rydell's On Golden Pond (bottom)
Generally speaking, most actors, actresses, and directors either get that once-in-a-lifetime Academy Award nomination, or, if they're lucky, a couple or a handful of nods within a period of two to fifteen years. There are a number of exceptions to that rule, of course, but in the Academy Awards' 83-year history very few performers and directors — 28 of them, to be exact — have received Oscar nominations after a gap of 20 years or more in their respective categories, i.e., actors/acting categories; directors/Best Direction category.
On the next page (please see link below) you'll find a complete list of Oscar's major "comebacks" — actors and/or directors who had to wait two decades or longer in-between nominations. Among those are record-holder Henry Fonda (41 years), John Huston, Julie Christie, Roman Polanski, Mickey Rooney, Alan Arkin, Lynn Redgrave, and Jeff Bridges.
Note: This is an updated version of a February 2006 post.